DMSO, a very polar solvent, is particularly suitable for formulating paint strippers generally in combination with a moderately polar cosolvent, such as an ether, a ketone, an ester, etc., which makes it possible to maintain the efficacy of DMSO while reducing the cost of the mixture. Certain combinations lead to true synergistic results, which, depending on the type of paint and on its degree of ageing, are equivalent or similar to the efficacy of the standard combination: methylene chloride+methanol (Double Liaison, Physique et Chimie des Peintures et Adhésifs—No. 467-468-1995 <<Nouveaux décapants à base de DMSO®>>[Double Bond, Physics and Chemistry of Paints and Adhesives—No. 467-468, 1995: “Novel DMSO-based strippers”], J. P. Lallier. It may be noted that protic solvents, i.e. solvents comprising an X—H organic function, X possibly being O, N or S, such as water and alcohols, are not indicated as cosolvents.
However, one drawback of DMSO is its excessively high crystallization point, equal to 18° C., which leads to storage problems, especially when outdoors in winter, and to handling problems at low temperature.
It is known that water can lower the crystallization point of DMSO: a 92.5/7.5 (w/w) DMSO/water mixture has a crystallization point of 0° C. However, the addition of water to a DMSO-based formulation entails a certain number of problems relating to the formulation and to the application:                during formulation, the water associates very strongly with the DMSO, giving rise to a highly hydrophilic and polar phase, and demixing very often arises during the addition of the cosolvent, which is, itself, aprotic and less polar;        in the presence of water, DMSO often shows a loss of efficacy, DMSO-water combinations leading to molecular aggregates with a low rate of diffusion;        the presence of water often results in thickening problems, in particular with cellulose-based thickeners.        
Certain monoalcohols also have a good capacity for reducing the crystallization point of DMSO. In general, these alcohols of petrochemical origin are hazard-labelled as being irritant, harmful or toxic and introduce flammability, which is detrimental with respect to DMSO that is free of any hazard labelling.
Formulators are thus currently obliged to use a special installation for heating DMSO drums or to store them and use them in heated premises.
The Applicant Company has sought to solve this problem, and it has found that the addition of diols and/or triols in a content less than the content of a cosolvent in the DMSO, which is usually from 50 to 90 parts by weight of cosolvent in a DMSO+cosolvent mixture, can very efficiently lower the crystallization point of DMSO. Thus, an 80/20 (w/w) DMSO+glycerol mixture has a crystallization point of 0° C.
Furthermore, and unexpectedly, this addition of diol(s) and/or triol(s) allows the production of a mixture that is miscible with the usual, less polar, cosolvents and does not impair the stripping efficacy of DMSO formulated with the diol and/or triol, in contrast with the case where DMSO is formulated with water.
The present invention is thus based on the principle:                of combining DMSO with an organic compound that contains several hydroxyl groups, promoting the lowering of the crystallization point of DMSO, and which is carbon-based, affording good miscibility with the other conventional organic constituents of stripping and cleaning compositions, such as solvents and activators, and avoiding demixing by moisture uptake (which takes place with water); and        of producing this combination with a diol and/or triol content that is high enough for efficient lowering of the crystallization point to take place, without, however, reaching the contents of a cosolvent, for which function an alcohol is not recommended since it impairs the stripping efficacy.        
Cryobiology (1978), 15(1), 93-108, Boutron P. and Kaufmann A. cites a DMSO+glycerol mixture (10/1 by weight) that is better than pure DMSO for avoiding the crystallization of a eutectic during slow cooling. The said publication does not mention that this mixture is advantageous for lowering the crystallization point of DMSO.
WO 03/048 393 A1 (PCT/IT 01/00611) describes the use of DMSO+glycerol mixtures in an entirely different application, namely the preparation of buffer solutions for the polymerase chain amplification of DNA.
JP 1986-308848 describes, for the microelectronics industry, as a photoresist stripper, a DMSO+propylene glycol mixture used at 100° C. Photoresist stripper formulations are often mixtures based on a dipolar aprotic solvent such as N-methylpyrrolidone or DMSO and a glycol ether. The latter solvent does not serve to lower the crystallization point of the active solvent when it is DMSO. In particular, U.S. Pat. No. 4,401,748, U.S. Pat. No. 4,401,747 and U.S. Pat. No. 4,395,479 mention mixtures based on NMP and diethylene glycol methyl ether. Patent JP 01042653 mentions mixtures based on DMSO and a diethylene glycol monoalkyl ether.
U.S. Pat. No. 5,798,323 and US 2001/0034313 A1 describe photoresist cleaning and stripping compositions based on a solvent and an alkanolamine; among the list of possible solvents are DMSO, ethylene glycol and propylene glycol. DMSO+diol mixtures are not illustrated.